Such cooling apparatus is widely used in industry, for example in the freezing of food. A liquefied gas, typically liquid nitrogen, may be used to cool food in a tumbler or screw conveyor, or may be used to freeze food in a tunnel or a so-called immersion freezer. In the example of a freezing tunnel, food is advanced on a conveyor through a chamber in the form of a tunnel into which liquid nitrogen is injected. Typically, the liquid nitrogen is directed at the food and extracts heat therefrom as it vaporises. A fan or fans are employed to provide a flow of cold nitrogen vapour through the tunnel in a direction opposite to that in which the food is advanced through the tunnel. This flow of cold nitrogen vapour is also able to extract heat from the food. In order to limit the amount of cold nitrogen vapour that spills out of the ends of the tunnel, a fan is employed to extract the cold nitrogen vapour from a position in the tunnel between its ends. The fan typically communicates with an outlet in the roof of the tunnel. Since liquid nitrogen vaporises at a temperature of - 196.degree. C., the temperature of the vapour extracted from the tunnel is well below freezing point even though there has been heat exchange between the vapour and the food (or other articles or material being advanced through the tunnel) and dilution of the nitrogen vapour with air takes place in the tunnel.
The operation of the exhaust fan induces a flow of air into the tunnel. Measures need to be taken to prevent the cold exhaust gas from causing ice to be deposited on the fan. Otherwise, there is a risk that either damage is caused to the fan, in operation, by the ice or that there will be a sufficient accumulation of ice to prevent its operation.
The solution normally adopted to this problem is to provide the ducting by which the fan is placed in communication with the outlet from the tunnel with an adjustable inlet for ambient air. Typically, this inlet is designed so as to enable the fan to draw in a flow of ambient air into the ducting at a rate three or four times that at which the mixture of cold nitrogen vapour and air enters the ducting from the outlet of the freezing tunnel.
There are a number of disadvantages associated with such exhaust gas extraction systems. In particular, the extraction duct needs to be of greater diameter than it would otherwise have to be in order to cope with the induced air flow. Moreover, the refrigerative capacity of the extracted nitrogen vapour is wasted. In addition, if the ambient air has been conditioned, a common practice in food processing factories, extracting air with the nitrogen vapour effectively reduces the overall efficiency of the air conditioning system. A further disadvantage is that practical problems arise with the control of the extraction system. The operation of the exhaust fan is typically linked to a valve controlling the flow of liquid nitrogen into the tunnel. Since the tunnel may be operated in association with a widely varying range of belt loadings, the temperature of the nitrogen vapour at the outlet can vary widely even though the valve is controlled so as to give a desired product temperature at the tunnel exit. Accordingly, in practice, difficulties can arise in continuously maintaining the fan free of ice even though the exhaust gas is considerably diluted with air.